1. Field of the Invention
This invention relates to braking force distribution control for a four wheel drive vehicle capable of conducting anti-skid control.
2. Description of Related Art
Conventionally, for a vehicle with a direct four wheel drive mode, when a brake pedal is stepped on to increase the hydraulic pressure of front wheels but decreasing the hydraulic pressure of rear wheels because of the braking force distribution control, a torque T1 acts on the front wheels to stop the wheel rotation while the counter torque acts on a front wheel axle and a rear wheel axle, thereby causing vibration in the drive train as shown in FIG. 7.
This vibration is considerably large and the continuous vibration delays the braking effectiveness resulting in a longer braking distance. This is disconcerting to the driver, and gives an adverse effect on the drive train including differential-related parts and an auto free hub.
It is an object of this invention to provide a method for braking force distribution control for a vehicle with a direct four wheel drive mode, which does not become an obstacle to an effective braking force and disconcerts the driver""s feeling.
With this invention a method for braking force distribution control for a four wheel drive vehicle is provided. The vehicle uses a direct four wheel drive mode for operating anti-skid control and braking force distribution control. The vehicle has a hydraulic unit comprising a main hydraulic circuit connected between a master cylinder and an inlet valve via a wheel cylinder and an auxiliary hydraulic circuit connected between the wheel cylinder and an auxiliary reservoir via an outlet valve. A wheel speed sensor determines the respective wheel speed, and an electronic control device controls the hydraulic unit, wherein the braking force distribution control for a vehicle during the direct four wheel drive mode, and under the condition that the anti-skid control is inhibited, does not operate pressure reduction control or limit the amount of pressure reduction.
With this invention, a method for braking force distribution control for a four wheel drive vehicle is provided. The vehicle uses a direct four wheel drive mode for operating anti-skid control and braking force distribution control. The vehicle has a hydraulic unit comprising a main hydraulic circuit connected between a master cylinder and an inlet valve via a wheel cylinder and an auxiliary hydraulic circuit connected between the wheel cylinder and an auxiliary reservoir via an outlet valve. A wheel speed sensor determines the respective wheel speed. An electronic control device controls the hydraulic unit, wherein the braking force distribution control during the direct four wheel drive mode and under the condition that the anti-skid control is inhibited operates pressure increase in the brake hydraulic pressure, or normal brake hydraulic pressure increase if vehicle vibration is detected.